1. Field of the Invention
The present invention relates to a color cathode-ray tube apparatus and, more particularly, an electron gun assembly for use in a color cathode-ray tube apparatus, which dynamically focuses electron beams, thereby forming a high-resolution image on the phosphor screen of the color cathode-ray tube apparatus.
2. Description of the Related Art
FIG. 1 is a cross-sectional view of a color cathode-ray tube apparatus of the most common type. As is shown in this figure, the color cathode-ray tube apparatus comprises a faceplate 3, a funnel 4, a neck 5, an electron gun assembly 6, a deflection unit 7, and a shadow mask 9. The faceplate 3 has an edge portion 3a. A screen 2 is formed on the inner surface of the face-plate 3. The funnel 4 connects the edge portion 3a of the faceplate 3 to the neck 5. The electron gun assembly 6 is located within the neck 5. The deflection unit 7 is shaped like a ring, surrounding the junction of the funnel 4 and the neck 5. The unit 7 is designed to deflect the electron beams emitted by the electron gun assembly 6. The shadow mask 9 is held in the faceplate 3 and opposes the screen 2, spaced apart therefrom by a predetermined distance. The mask 9 has a number of apertures 8 for guiding the electron beams onto the screen 2. The color cathode-ray tube apparatus further comprises an inner conductive layer 10 and an anode terminal (not shown). The layer 10 is coated uniformly on the inner surface of the funnel 4 and also on a part of the inner surface of the neck 5. The anode terminal (not shown) is mounted on a part of the inner surface of the funnel 4.
Red phosphor, green phosphor, and blue phosphor are coated on the screen 2 in the form of stripes or dots. The electron gun assembly 6 emits three electron beams BR, BG, and BB. The beams BR, BG, and BB are deflected by the deflection unit 7, guided by the shadow mask 9, and applied onto the phosphor stripes or dots. When excited by these electron beams, the red phosphor stripes or dots emit red light, the green phosphor stripes or dots emit green light, and the blue phosphor stripes or dots emit blue light.
The electron gun assembly 6 has a beam-forming section GE and a beam-processing section ML. The section GE generates three parallel electron beams BR, BG, and BB in so-called "in-line alignment," and accelerates and controls these beams. The beam-processing section ML focuses and converges the three electron beams emitted from the beam-forming section GE. The electron beams BR, BG, and BB emitted from the electron gun assembly 6 are deflected by means of the deflection unit 7, guided by the shadow mask 8, and applied onto the screen 2. Hence, the electron beams scan the screen 2, forming rasters on the screen 2.
The deflection unit 7 has a horizontal deflection coil and a vertical deflection coil. The horizontal deflection coil generates a horizontal-deflection magnetic field for deflecting the electron beams in the horizontal direction. The vertical deflection coil generates a vertical-deflection magnetic field for deflecting the electron beams in the vertical direction.
When any beam emitted from the electron gun assembly 6 is deflected by means of the deflection unit 7, it cannot be correctly converged and thus fails to form a beam spot on the target phosphor stripe or dot formed on the screen 2. To converge the beam with accuracy, the so-called "convergence-free system" is used in the conventional cathode-ray tube apparatus. In this system, the horizontal-deflection magnetic field is formed into a pincushion-shape, and the vertical-deflection magnetic field is formed into a barrel-shape. The pincushion magnetic field and the barrel magnetic field act, in concert, on the three electron beams such that the beams are correctly converged on the target phosphor stripes or dots, respectively.
Generally, even a magnetic field, which is considered to be uniform in its intensity distribution, includes a small pincushion component or a small barrel component. FIG. 2A schematically shows a magnetic field including a pincushion component. An electron directed to the peripheral portion of the screen 2, in particular, is subjected to a relatively prominent deflection aberration as the beam passes through this magnetic field. Consequently, when the beam lands on the peripheral portion of the screen 2, it forms a beam spot which is distorted as is shown in FIG. 2B. The distorted beam spot consists of a horizontally elongated core having high luminance and halos having low luminance, one extending upward from the core and the other extending downward from the core. The larger the cathode-ray tube apparatus, or the more the beam is deflected, the more the beam spot is distorted.
This distortion of the beam spot is produced due to over-focusing of the electron beam in the vertical plane. A method of reducing or eliminating the deflection aberration, i.e., the cause of the distortion of the beam spot, is disclosed in Television Technology, Vol. 36, pp. 41-55, 1988. This method is characterized in that a quadruple lens is incorporated into an electron gun assembly, and is driven to emit an electron beam having a cross section whose upper and lower portions are more intense than the right and left portions. When this method is applied, however, an electron beam will have an elliptical cross section extending in the vertical direction, and will be subjected to a more prominent aberration. Thus, in order to focus the electron beam appropriately, the power of the electron lens must be changed greatly. Here arises a problem. The more the power of the lens is varied, the greater the changes in the voltage for achieving dynamic focusing of the beam, and, hence, the greater circuit load the cathode ray tube apparatus requires.
Further, in the quadruple lens, the electron beam is excessively diverged in the vertical plane and the electron beam is excessively focused in the horizontal direction. It is therefore necessary to add to the lens some elements for correcting this over-focusing of the electron beam, which would render the lens more complex in structure. To control such a complex electron lens, the circuit for controlling the electron gun assembly needs to be complex inevitably.
Japanese Laid Open Patent Application No. 60-22140 discloses a cathode-ray tube apparatus, wherein electron beams are guided to cross twice the axis of the electron gun assembly, thereby to achieve a sufficient resolution even if the beam current is comparatively small. The gun assembly used in this apparatus comprises a three-electrode unit including a first grid G1 (i.e., the control electrode) and a second grid G2 (i.e., the shield electrode), a main lens electrode for forming a main electron lens, and an auxiliary electrode G2s. The electrode G2s is interposed between the three-electrode unit and the main-lens electrode, and is applied with a voltage which is lower than the voltage applied to the second grid G2 and changes in accordance with the desired deflection angle of the electron beam.
In this electron gun assembly, the electron beam crosses the axis of the assembly twice until it reaches the main lens electrode, and its peripheral portion is trimmed by a trimming electrode as the beam travels from the main lens electrode to the phosphor screen. The beam, however, forms but a distorted spot on the phosphor screen due to the deflection aberration, though the image resolution is sufficiently high if the beam current is relatively small. This is because the beam is anisotropically distorted by the deflection magnetic field, and the anisotropic distortion cannot be eliminated since the beam crosses the axis of the gun assembly two times while traveling from the cathode to the main lens electrode. Moreover, even if the second cross-over is dynamically shifted on the axis of the gun assembly, the shape of the second cross-over is changed in the horizontal or vertical plane, due to the auxiliary electrode G2s which are located between the cathode and the third grid G3, or within the beam-forming section of the gun assembly. Hence, the deflection aberration cannot be either reduced or eliminated in the cathode-ray tube, wherein self-convergence deflection magnetic fields are generated. Rather, the deflection aberration increases, and the beam will form an even more distorted spot on the phosphor screen.
The electron lens is located in the beam-forming section of the gun assembly, in order to make the beam cross the axis of the gun assembly for the second time. This electron lens comprises four thin electrodes. These electrodes are located so close to one another that their potential affect mutually to a degree which depends on the shapes of the electrodes and also those of the openings made in the electrodes. Consequently, the characteristics of the electron lens fluctuate. Due to the fluctuation of its characteristics, the lens can hardly focus an electron beam sufficiently in the vertical direction only. Rather, this quadruple lens may focus an electron beam more in the horizontal direction than in the vertical direction.
As may be clear from the above, the larger the color cathode-ray tube apparatus, or the more the electron beam are deflected, the more the resultant image will be deteriorated.